1. Field of the Invention
This invention relates to systems and methods for improving the quality of video images generated by a forward-looking infrared (FLIR) sensor array. These systems and methods combine the means and method steps of scene-restored and DC-restored systems and methods, and include an adaptive clamp level which can vary over a single image, rather than a clamp level which is constant for a single image.
2. Description of Related Art
Until now, image restoration systems used DC restoration alone or scene restoration alone to generate video images based upon information received from sensors. In the case of images obtained from FLIR sensors, a DC restoration system provides good ground/sky distinction, but lacks local area contrast. DC-restored video images from FLIR sensors, though suitable for a pilot following "Nap of the Earth" flying techniques, lack the definition necessary to detect targets against a uniform background such as the sky or the sea.
By contrast, a scene-restored system provides images that are highly uniform and have high local contrast, but can include horizontal artifacts, and have poor ground/sky distinction. Such a system is suitable for target detection, but not for "Nap of the Earth" flying techniques.
A common FLIR sensor system includes a vertical array of detectors, which is scanned horizontally across a scene to form a complete FLIR image. The detectors in the array are AC-coupled, meaning that any DC component of the FLIR image is lost. To restore the DC component, the detector array scans a uniform temperature source, which permits subsequent image processing to restore the lost DC component.
As an example, FIGS. 1, 2 and 3 show plots of an average scene output and an average source output for each detector in a vertical array of detectors in a FLIR system. The image used for these graphs is a typical, airborne FLIR image, which includes sky, ground and horizon.
FIG. 1 shows a DC-restored image, meaning an image that is the same as the original scene. The left-hand side of FIG. 1 shows the sky, which is cold, and the right-hand side of the graph, the ground, which is warm. The dip at the center of the graph occurs at the horizon. In FIG. 1, the average source output is constant for each detector because the source is at a uniform temperature. The image seen by the FLIR system is restored by adding an offset value to the output of each detector to force the source output to be equal to a constant, called the Clamp Level. The overall dynamic range for such an image is large, and thus difficult to map to the limited number of gray shades available with a display device such as the heads-up display devices.
FIG. 2 is a graph made from the same scene as FIG. 1, but with the image scene-restored instead of DC-restored. The average scene output is constant for each detector, and shows no distinction between ground and sky, and no definition at the horizon. The average temperature source is not constant, though all detectors scan the same uniform temperature source. The overall dynamic range is smaller in FIG. 2 than in FIG. 1, and is easier to map to a limited number of gray shades.
Until now, no system has provided the advantages of both a scene-restored and a DC-restored system, though such a system is highly desirable.